Screw fasteners are generally manufactured by cold heading methods in which steel wire is transformed by pressure in a die to form a screw blank. Typically the cold heading process involves impacting the wire in a blank forming die. Such dies are typically made from hard steel such as carbide steel. For a standard screw fastener with a straight shank such dies will be a unitary die having a straight bore formed by electric discharge machining or drilling with a diamond tip drill. These dies typically have an operating life of about 200,000 pieces.
However, where a fastener blank with a reduced, asymmetric or offset tip or other section is desired, these unitary dies have a much more limited operating life, due to the stresses on the surfaces of the die which form the tip section. In particular, the surfaces which are other than parallel to the direction of the impact forces will be most subject to repetitive wear and tear, both from the shock of the extrusion impact and abrasion as the wire is extruded through the tip section forming area of the die. These specialized dies often have a limited operating life of as few as about 20,000 pieces.
To enhance the operating life of such a die, it would be desirable to apply a hard coating to the particular surfaces of the die which are subject to stress and wear. However, it is often extremely difficult to selectively apply such a coating only to the selected areas, particularly tip section forming areas, because they are located deep within the die. It is to be appreciated that the bore of a die will often be small in diameter and very deep relative to the bore diameter and it is difficult to introduce coating materials into the bore and cause them to coat the tip forming areas in the bottom of the bore.